Placing a geosynchronous satellite into orbit typically involves three principal mission phases. First the satellite is placed in low earth orbit not far above the earth's atmosphere, either as part of the payload of a space shuttle vehicle or on a conventional non-reusable rocket vehicle. In the second phase, the satellite orbit has its apogee or highest point raised in altitude by one or more rocket "burns" at a selected point in the orbit, until the apogee is approximately at geosynchronous altitude. Finally, the satellite is given an apogee "kick," i.e. a further rocket burn at apogee that circularizes the orbit at geosynchronous altitude.
Once in orbit, rocket engines are called upon for three additional functions: station keeping, station changes and attitude control, which are sometimes referred to collectively as reaction control system (RCS)functions. Satellites are usually required to maintain a particular "station" with respect to the earth's surface. Moreover, to satisfy requirements of a particular mission, satellites are sometimes required to change from one station to a different station. Changing the satellite to another station obviously requires expenditure of energy. Maintaining a given station also requires the expenditure of energy, even though the orbit is theoretically self-sustaining and geosynchronous. Various factors that create drag and reduce or change the satellites velocity, such as the non-spherical nature of the earth, the gravitational influences of the moon and sun, and so forth, require that the orbit be corrected from time to time if the required station is to be maintained. To make either station keeping corrections or station changes, the station keeping/changing rocket engines provide a "burn" sufficient to slightly change the satellite's velocity. Attitude control is simply the use of multiple rocket engines on the spacecraft to maintain a particular angular attitude or "pointing" of the vehicle. This may be needed, for example, to point an antenna or other sensor at the earth, the sun, or a star.
Unfortunately, the rocket engine performance characteristics required for the various functions of orbital transfer, station keeping/changing and attitude control are not identical.
A figure of merit often used in the comparison of the efficiency of rocket engines is the specific impulse, I.sub.SP, which is defined as the thrust developed by the engine per unit of propellant weight flow rate. If the thrust is measured in pounds and the flow rate in pounds per second, the units for the measurement of specific impulse are seconds. The specific impulse is somewhat analogous to a miles-per-gallon figure for an automobile, since it measures how much thrust is developed for a unit fuel flow rate.
Another measure of performance is, of course, the thrust force generated by the engine. For the rapid acceleration that is required in a transition to geosynchronous orbit, particularly at the apogee "kick" phase of a mission, an engine with a relatively large thrust is required, perhaps generating up to several thousand pounds of thrust force. The specific impulse, Isp, of such a high thrust engine is also important, and should be in the 300 to 400 second range.
For station keeping and attitude control, high thrust is far less important, since most station-keeping and attitude control maneuvers can be efficiently accomplished with low-thrust burns of the rocket engines. However, fuel economy is very important for rocket engines used in these activities. Hence, the higher the I.sub.SP, the better. Present monopropellent rockets for these functions achieve an I.sub.SP on the order of about 225 to 235 seconds.
Because of the different requirements, earlier propulsion systems involved using multiple fuels and engine systems for the apogee kick and the velocity and attitude control. For example, a solid rocket was used for the apogee kick engine and hydrazine catalytic engines were used for the station keeping/changing velocity and attitude control system thrusters. There is nothing inherently incorrect with that traditional approach, except that the use of two separate propulsion systems weighs more, thereby severely limit the size of the useful payload that can be placed and maintained in orbit, and it costs more.
Some improvement can be obtained using an integrated bipropellant system, in which both the apogee kick engine and the RCS thrusters each use a bipropellant fuel system, such as monomethyl hydrazine (MMH) as the fuel and binitrogen tetroxide (N.sub.2 O.sub.4)as the oxidizer. Even with that, there is still room for further improvement in the payload that can be placed in orbit for a given mission. Another way to look at the matter is that there is room for improvement in the lifetime that a given spacecraft payload could be maintained in orbit. With a more efficient propulsion system, a greater payload may be maintained in orbit for a given time, or the same payload may be maintained in orbit for a longer time.
To that end, additional propulsion systems were proposed to increase payload efficiency. In U.S. Pat. No. 5,282,357 granted Feb. 1, 1994 to one of the present inventors and owned by the same assignee, a spacecraft rocket propulsion system is disclosed which uses the same fuel for both a bipropellant rocket engine capable of producing high thrust to provide the apogee kick, and one or more monopropellent rocket engines that deliver low thrust, such as the MRE-1 thrusters, for the station keeping and attitude control functions. By employing a common fuel for both the bipropellant and monopropellant rocket engines, the spacecraft is required to stow only one fuel, the Hydrazine, as example, and that reduces weight in comparison to prior systems requiring different fuels and storage vessels, thereby improving propulsion efficiency.
Further, in additional patents U.S. Pat. No. 5,417,049 granted May 23, 1995 and U.S. Pat. No. 5,572,865, granted Nov. 12, 1996, related to the foregoing '357 patent, and issued to one or more of the present inventors, among other things, a new propulsion system is proposed and a new bi-propellant thruster construction is described that has dual mode capability. That thruster construction is presently referred to as a Secondary Combustion Augmented Thrusters or, simply, as a "SCAT" thruster.
A propulsion system is also there presented that employs a bipropellant engine to provide the high thrust apogee kick and the bipropellant SCAT thruster to provide station keeping and attitude control functions. In keeping with the description of '537 patent, both thrusters in that propulsion system use a common fuel, Hydrazine. The bipropellant thruster uses the liquid oxidizer, N.sub.2 O.sub.4, as the second propellant.
The SCAT thruster uses the same oxidizer to both cool the thruster chamber, whereby the oxidizer is transformed to a gaseous phase, and, in the gaseous phase, as the second propellant for the bipropellant mode of operation. In its construction, the SCAT thruster contains two connected reaction chambers. Liquid propellant fuel, such as hydrazine, is fed into the first chamber where it reacts with a catalyst and enters the gaseous phase in an exothermic reaction, thereby heating the chamber walls, and the reaction propellant gas is propelled by the reaction into a second chamber. Liquid propellent oxidizer is fed through a heat exchanger surrounding the thruster and thereby cools the unit, and the associated energy absorbed by the oxidizer transforms the liquid oxidizer to the gaseous state. The gaseous oxidizer is then routed into the second chamber and mixes with the gaseous propellant fuel entering from the first chamber and reacts with the propellant to create thrust.
Using the two propellants, the SCAT thruster produces an I.sub.SP of about 315 to 325 seconds and a thrust .DELTA.V that is significantly greater than that available from monopropellent RCS thruster. For additional details of construction of the SCAT thrusters, the reader may make reference to the afore cited patents. Such SCAT thrusters are commercially available from TRW Inc., Redondo Beach, Calif., assignee of the present invention.
Using the thermal energy to perform the work of vaporizing the liquid oxidizer instead of the alternative of radiating that thermal energy into space and using an alternative vaporization procedure for the oxidizer obviously somewhat enhances the efficiency of the propulsion process used in the SCAT thruster, an advantage to that engine.
The cooling effect inherent in the SCAT thruster's bipropellant mode of operation raises an additional factor of importance for some spacecraft missions: durability. Heating to high temperatures, particularly to temperatures close to the engine metal's breakdown or melting temperature is corrosive of metals and, if possible, is best avoided. Rockets for velocity and attitude control application are used repeatedly over a mission that may last ten years or more and, therefore, engine durability is important.
Cooling the small monopropellant engines typically used for reaction control functions is difficult due to the engine's small thermal radiating surfaces. Any prolonged use may raise the temperature enough to damage the thrust chamber. Thrust chamber durability can be improved by using more exotic materials, such as Columbium, which generally withstands the four to five thousand degree Fahrenheit temperatures generated in the combustion chamber. That alternative, however, significantly increases construction costs, a decided disadvantage.
Cooling the thrust chamber reduces thermal stress on the metal that forms the chamber walls. The lower operating temperature lessens the need for use of exotic materials and coatings. For a given metal, the metal is more durable at lower temperature and, hence, the engine will last longer. The inherent cooling that occurs in the normal operation of the SCAT engine suggests a longer operational life in comparison to other engines, all other factors being equal, an additional advantage to the engine.
The SCAT thruster is noted as having dual mode capability. It is shown to operate in a bipropellant mode and, alternately in a monopropellent mode. It is now realized that a simple SCAT bi-modal thruster can operate in either a bipropellant mode and achieve an I.sub.SP of over 315 seconds, or, alternatively, can operate in a monopropellant mode and achieve an I.sub.SP of about 225 seconds.
The SCAT thruster is described in the two cited patents as useful for station keeping/changing and attitude control functions of the propulsion system, and the patents thereby propose the SCAT thruster as a substitute for the monopropellant RCS thrusters in the single fuel rocket propulsion system earlier described in the '357 patent, in which those thrusters serve as a companion to the high thrust apogee kick rocket engine. For one, the substitution reduces propellant weight and thereby increases the payload size carried on a mission.
Despite the apparent advantage of the dual mode SCAT engine, Monopropellant rockets, such as the wide range of catalytic thrusters available today in the industry, despite their lower performance, remain the engine of choice for station keeping and attitude control functions in satellite systems. The designs of those monopropellant rockets are space proven; its performance is predictable, it has been reliable; and it offers none of the uncertainty of a new product for the satellite designer. Those advantages apparently outweighed the superior performance offered by the SCAT thrusters. The requirements for many space flight missions offered neither motive or incentive to change to another rocket engine for station keeping and attitude control application and the lack of industry acceptance suggests the proposed substitution as overly ambitious and/or illusory advantage.
Notwithstanding such discouragement, an important aspect of the present invention is a propulsion system that relies on the SCAT engine as the important component, providing weight savings with no significant compromise in reliability and hardware compatibility, and offering an advantage of two modes of operation, but one which does not eliminate RCS monopropellent rockets. The present invention retains the benefit and advantage of the monopropellent thrusters and adds the advantage of SCAT thrusters in satellite propulsion systems.
Accordingly, an object of the present invention is to provide a novel, highly reliable and more efficient propulsion system suitable for geosynchronous and other high-energy mission spacecraft programs.
And an additional object of the invention is to provide a simple means to obtain high performance by converting a monopropellant system to a dual mode monopropellant and bipropellant system that avoids the extreme complexity and cost increase attendant to binitrogen tetroxide--hydrazine type bipropellant systems.